The regional cerebral metabolic rate for glucose (rCMRglc) was examined, as a measure of cerebral functional activity in healthy men at different ages, and in patients with primary progressive dementia of the Alzheimer's type. and adult autism. rCMRglc was determined by means of positron emission tomography (PET) with 18-F-2-deoxy-D-glucose, under resting (unstimulated) conditions, when the subject's eyes were covered and his ears plugged to reduce sensory input. In 21 healthy men between the ages of 21 and 83 years, average hemispheric glucose utilization and glucose utilization in individual regions of the right and left hemispheres, did not decline significantly with age (p greater than 0.05). In 4 young adult subjects with Down syndrome (19-27 yr.), rCMRglc as measured with PET was elevated by 20-40% as compared with age-matched healthy controls, indicating that young adult Down syndrome subjects use glucose excessively throughout the brain despite their retardation. Similarly, adult patients with autism, an irreversible psychiatric disorder with onset in infancy and with a suspected neurological basis, showed elevated values of rCMRglc, to values between normal and Down's syndrome subjects. Adult patients with Alzheimer's disease showed variable reductions in rCMRglc, which were distributed in the frontal parietal and temporal lobes, and which were correlated to some extent with measured localized cognitive deficits, as determined with neuropsychological testing.